filetransfer sample doesn't use bufferedAmountLow

[fippo]

https://github.com/webrtc/samples/blob/gh-pages/src/content/datachannel/filetransfer/js/main.js
doesn't use the bufferedAmountLow callback. It should, like the datatransfer one does.

cc @taylor-b :-)

[ErikHellman]

Since we're using a FileReader and the load event from that, it becomes tricky to also use bufferedamountlow event.

I'm closing this since the sample works well at the moment and there is no need to use this event. Also ,there seems to be some problems with that event at the moment.